Burner with steady atomization at low back pressure

ABSTRACT

An apparatus for fuel preparation ( 100 ) for a burner ( 18 ) for regenerating a particle filter ( 16 ) of an exhaust gas aftertreatment system comprising an exhaust gas passage ( 14 ) for discharging a hot exhaust gas flow from an internal combustion engine ( 12 ), in particular a compression-ignition internal combustion engine, is proposed. The apparatus for fuel preparation ( 100 ) has an atomizing device ( 110 ) for preparing the fuel, a combustion chamber ( 116 ), an ignition device ( 26 ) and a combustion air supply ( 24 ). The combustion air supply ( 24 ) provides a primary air feed line ( 112 ) for preparing the fuel and a secondary air feed line ( 114 ) for feeding combustion air in order to produce an ignitable fuel/air mixture.

BACKGROUND OF THE INVENTION

The increasingly stringent legal requirements for protecting publichealth and the environment, which regulate the emissions of motorvehicles, represent a challenge for the development of engines and forthe exhaust gas aftertreatment technologies. The reduction of particleemissions thus becomes particularly important and in this respect aboveall emissions of compression-ignition internal combustion engines, whichhave among other things a carcinogenic effect. Hence, many methods anddevices have been developed to remove the soot particles that arepresent in exhaust gases. The use of particle filter systems forcapturing pollutants present in particle form is well known.

In order to carry out particle separation, exhaust gases are passedthrough a heat resistant filter medium. The particles captured by thefilter medium, which especially have diameters in the range of 10 to5000 nm, coat the filter and thus lead to an increase in the pressureloss across the filter. This makes it necessary to continuously orcyclically perform a regeneration of the filter medium. The regenerationgenerally takes place by means of a practically residue-free combustionof the accumulated soot particles using oxygen. Ignition temperatures ofapproximately 600E C are however required for particle combustion, saidignition temperatures being only seldom achieved in the exhaust gas of acompression-ignition internal combustion engine even under full loadconditions. For this reason, it is known to provide a burner in order toobtain a corresponding increase in the exhaust gas temperature.

A catalytic burner is known from the German patent publication DE 100 24254, which is disposed between the internal combustion engine and theparticle filter and increases the exhaust gas temperature without anignitable fuel mixture having to be provided. The fuel injecteddownstream of the engine by the catalytic burner is converted at anoxidation catalyst while reaction heat is being released, wherein theexhaust gas temperature is raised to the level of a regenerationtemperature. It is however required for the temperature of the exhaustgases to have already reached a certain value so that the catalyticreaction can take place at the oxidation catalyst. Below thistemperature, the activity of the catalyst is so low that the injectedfuel is not sufficiently converted. A regeneration of the particlefilter is not reliably possible in each engine load range. Furthermore,an additional injection of fuel is required, which is only an option incompression-ignition internal combustion engines having common railtechnology, wherein an extensive intervention into the engine managementsystem is necessary.

The use of a combustion device in the filter apparatus is likewise knownfor producing the required temperature in the exhaust gas flow. Acombustion device is thus known which comprises a combustion chamber anda burner, wherein metered fuel and metered combustion air are mixed toform an ignitable fuel/air mixture. The fuel/air mixture formed in thecombustion chamber is ignited by an ignition device. The hot combustiongases are mixed with the exhaust gases and raise the exhaust gastemperature to such an extent that the soot particles accumulated in theparticle filter oxidize. In general, this process relates to aforced-air burner comprising a pressure-atomizing nozzle.

The burner capacity required to sufficiently heat the exhaust gas flowis generally a function of the engine operation, wherein the burnercapacity is adapted to the current operating point of the internalcombustion engine. If the entire exhaust gas flow is brought to therequired burn-off temperature, a high burner capacity is required whichleads to a higher fuel consumption. A large quantity of air is thusrequired in order to achieve an ignitable fuel/air mixture; thusrequiring a larger burner. The air supply, which substantially ensuesfrom a compressed air reservoir or via an electrically driven air pump,thus puts a significant load on the on-board electrical system of thevehicle as a result of the relatively high power consumption of theassociated electric motor. This can lead to malfunctions at lowrotational speeds of the combustion engine. On the other hand, acorrespondingly increasing flow velocity in the air feed line occurs asa result of the increased quantity of air, whereby an increased backpressure is produced. The disadvantage of this result is that anincreased back pressure leads to an increase in the electrical powerconsumption. In the case of low burner capacity, the danger also existsthat the preparation of the fuel is insufficient. When the fuel isinsufficiently prepared or respectively atomized, uncombusted fuel canreach the particle filter and destroy the same.

The German patent publication DE 10 2009 051 327 A1 describes a methodfor controlling the temperature of combustion air of an oil burner. Thefeed of combustion air to a mixing zone of the burner can result as afunction of an operating mode by means of a first and/or a first and asecond feed pathway, wherein heat in each case is supplied to thecombustion air.

Atomizing burners for liquid fuels are likewise known from the Germanpatent publication DE 38 26 446 A1, wherein heavy fuel oil comprising aprimary air flow is fed in a nozzle tube, which is surrounded by acasing tube, over which secondary air is fed.

SUMMARY OF THE INVENTION

According to the invention, an apparatus for fuel preparation for aburner for regenerating a particle filter of an exhaust gasaftertreatment system of an internal combustion engine comprising acombustion air supply is proposed, wherein the combustion air supplyprovides a primary air feed line for preparing the fuel and comprises asecondary air feed line that supplies combustion air in order to producean ignitable fuel/air mixture.

In a particle filter of an exhaust gas aftertreatment system based onknown filter bodies, for example: filter channels, filter meshes orfilter pockets from ceramic or sintered metallic material, particlesaccumulate over time. The particle accumulations can be pronounced tosuch an extent that the exhaust gas flow through the exhaust gasaftertreatment system is limited or even blocked. This leads to anincreased back pressure and finally to a decrease in performance,increased exhaust gas temperatures and to an increase in fuelconsumption. For that reason, the particle filter is either continuouslyor periodically regenerated with hot air during operation.

According to the invention, the temperature required in the exhaust gasflow for regenerating the particle filter is produced by a combustiondevice, in particular a burner comprising a combustion chamber and anatomizing device, wherein the combustion air is supplied to the burnervia a primary and a secondary air feed line. A primary air mass flow,which can be supplied largely independently of the burner capacity, ismade available to the fuel preparation at the atomizing device via aprimary air feed line. In the case of increased burner capacity and theensuing increased air requirement, the air supply takes place aside fromthe primary air feed line further via the secondary air feed line,wherein the back pressure can be held substantially constant at a lowlevel at the atomizing device.

According to the invention, a primary air mass flow is supplied via theprimary air feed line to the atomizing device, which finely atomizes theinjected fuel. An aerosol which is ignitable is obtained immediatelydownstream of a nozzle by an intensive mixing of the fine fuel mist withthe combustion air. When the air volume requirement is increased, asecondary air mass flow is conveyed via the secondary air feed line. Theadditional air feed line facilitates a setting of a suitable fuelinjection ratio, or fuel/air ratio, can influence the injectiondirection and produces a correct size of the fuel droplets. In addition,the back pressure can be held low and therefore a mechanical andelectrical power requirement, for example of an air pump, can be heldsubstantially low.

In a preferred embodiment, the secondary air feed line opens outdirectly into the combustion chamber, the fine aerosol consisting offuel mist and combustion air, which is produced by the atomizing device,mixing thereby with the secondary air mass flow. This arrangement can bevery easily configured so that the costs of this apparatus can be keptlow. In addition, the back pressure in this arrangement remains at a lowlevel because no throttling point is provided in the secondary air feedline.

In an alternative embodiment of the apparatus for fuel preparation for aburner in an exhaust gas aftertreatment system, the secondary air massflow supplied by the secondary air feed line enters immediately into theatomizing device; and therefore a support of the atomization of theinjected fuel is obtained. The secondary air mass flow of the atomizingdevice can thus be supplied in a manner which allows the atomization ofthe injected fuel to be influenced and substantially improved withregard to the size of the fuel droplets that is achieved and thehomogenization of the fuel/air mixture. Furthermore, influence can beexerted over the flame produced at the burner, for example by firstlysubjecting the emerging flame to a swirl and if need be secondly to aconstriction; and in so doing, a flame stability is achieved even whenthe air ratios continue to deviate from the target value. By means ofthe secondary air mass flow, contamination at the atomizing device, inparticular at the nozzle, can be held to a minimum so that fine mistingnozzles can also be used in the atomizing device. The nozzle cantherefore be cleaned and cooled down by means of the air feed line.

The atomizing device can, for example, have a nozzle tube, which issurrounded by a casing tube. Lateral openings are provided on the casingtube for introducing the secondary air mass flow into the formed annularspace. A certain alignment of the openings can ensure that via thesecondary air mass flow the flame is subjected to a swirl and/or aconstriction, wherein a stabilization of the flame is achieved. Aconstriction of the air mass flows introduced produces a high flowvelocity. A high flow velocity of the primary air mass flow or acorrespondingly large difference in velocity with respect to the fuelflow can be used to atomize the fuel. A high difference in velocitybetween the primary mass flow and the secondary mass flow promotes theaerosol formation, the difference in velocity being especially great ifboth air mass flows are subjected to a swirl having a differentorientation.

The metering of the combustion air is generally controlled via a controldevice. For example, the air supply occurs via a magnetic valve from acompressed air reservoir or from an electrically driven air pump, forexample via a rotary vane pump, claw pump or an eccentric rotor pump. Inan alternative embodiment of the apparatus according to the invention, apressure regulating valve is disposed in at least one of the air feedlines, preferably in the secondary air feed line. The pressureregulating valve in the air feed line can be embodied as a simplemechanical pressure valve or as a controllable pressure regulatingvalve. The option therefore exists to feed the combustion air, startingat a pressure in the air feed line to be determined, via the primary andthe secondary air feed line to the burner. In so doing, the backpressure, which rises when the air volume is large, is limited. Theatomization of the injected fuel can thereby take place under optimizedconditions and the stability of the flame is achieved in a simple andcost effective manner.

During a start-up phase of the burner, in which only a small volume ofair is required, the prevailing pressure lies below the opening pressureof the pressure regulating valve that is disposed in at least one airfeed line. The air volume is only provided via one air feed line of theapparatus for fuel preparation. As the power output of the internalcombustion engine increases, the power requirement of the burnerincreases; and therefore the required air volume rises, the backpressure increases, and the air volume is supplied via the primary andthe secondary air feed line after the opening pressure of the at leastone pressure regulating valve has been exceeded.

The switching of the air supply from one to two air feed lines can bearbitrarily controlled, wherein the conditions at a low burner capacityas well as at a high burner capacity can be taken into account.

Provision is made in a further embodiment of the apparatus for fuelpreparation for a burner for the fuel to be injected to be mixed withthe air, in particular with the primary air mass flow supplied via theprimary air feed line, upstream of the atomizing device and for thismixture to be delivered to the combustion chamber. For example, the fuelmass flow and the air mass flow are brought together at a junction ofthe lines, for example in the form of a T-piece. The combustiblefuel/air mixture generated in this way enters premixed into theatomizing device. In addition, a reliable ignition of the fuel/airmixture is assured because the mixture is not generated in thecombustion chamber but outside of the same. In so doing, an ignitablemixture having a specified air/fuel ratio can be set. In thisembodiment, it is furthermore possible to combine several necessarycomponents, for example: metering valves, check valves and/or pressureregulating valves, into one component.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are depicted in the drawings andexplained in detail in the following description.

In the drawings:

FIG. 1 shows an exhaust gas treatment system according to the prior art;

FIG. 2 shows an exemplary embodiment of an apparatus according to theinvention for fuel preparation for a burner comprising a secondary airinlet into a combustion chamber;

FIG. 3 shows and exemplary embodiment of an apparatus according to theinvention for fuel preparation comprising a secondary air inlet into amixing zone;

FIG. 4 shows an alternative exemplary embodiment of the apparatusaccording to the invention for fuel preparation for a burner.

DETAILED DESCRIPTION

An exhaust gas aftertreatment system 10 according to the prior art isdepicted in FIG. 1 in a strongly schematized representation.

Exhaust gases from an internal combustion engine 12 arrive at a particlefilter 16 via an exhaust gas duct 14 after passing through a burner 18connected in between, wherein the burner 18 and the particle filter 16are disposed consecutively in a housing 20 in the direction of flow ofthe exhaust gases. Soot particles, which are carried out with theexhaust gas, accumulate in the particle filter 16. Depending upon theselected mode of operation, the fuel/air mixture generated by the feedof fuel 22 and air 24 is ignited at the burner 18 via an ignition device26. The hot gases exiting the burner 18 mix with the exhaust gases in anantechamber 28 of the particle filter 16 causing an increase in theexhaust gas temperature such that the soot particles accumulated in theparticle filter 16 oxidize. The regeneration of the particle filter 16occurs as a rule during the operation of the internal combustion engine12, wherein the residual oxygen in the exhaust gases can be used for theoperation of the burner 18. The combustion air is delivered via anelectrically operated air pump 30 which is driven by an electric motorthat is not depicted. The electric motor is connected to an on-boardbattery, which is charged by the alternator (not depicted).

Embodiment Variants

An exemplary embodiment of an apparatus according to the invention forfuel preparation 100 for a burner 18 is depicted in FIG. 2.

The apparatus for fuel preparation 100 comprises an atomizing device110, with which injected fuel is prepared in a suitable manner, inparticular is atomized. Fuel is delivered to the atomizing device 110via the feed 22, wherein a magnetic valve or a fuel delivery pump is,for example, provided for metering the fuel, said magnetic valve or fueldelivery pump being driven by a controllable electric motor (notdepicted). The fuel can be extracted from a fuel reservoir of theinternal combustion engine 12 via an intake line, wherein the deliveryrate can be changed by varying the rotational speed of the electricmotor. According to the invention, combustion air is supplied to theburner 18 via a primary air feed line 112 and a secondary air feed line114. The metering of the air feed takes place, for example, via amagnetic valve or via an electrically operated air pump (not depicted).The fuel which was fed is finely atomized at the atomizing device 110,which, for example, is designed as a swirl burner comprising atangential fuel and/or air feed, an ignitable fuel/air mixture beingthereby formed. The atomizing device 110 of the burner 18 can, forexample, be constructed as an annular nozzle, the fuel sweeping alongthe annular atomizing tongue thereof and being finely atomized by arotating air flow. The aerosol produced, which enters into a combustionchamber 116, is normally ignited by an ignition device 26. The use ofignition electrodes, pencil-type glow plugs or a glow unit is known.Besides the combustion air supplied as the primary air mass flow via theprimary air feed line 112, a required additional quantity of compressionair is inventively delivered during an increase in burner capacitydirectly into the combustion chamber 116 via the secondary air feed line114 pursuant to the exemplary embodiment depicted in FIG. 2.

A pressure regulating valve 118 is disposed in the secondary air feedline 114. Said pressure regulating valve 118 can be embodied as a simplemechanical pressure regulating valve, which unblocks the secondary airfeed line at a specific opening pressure. The pressure regulating valve118 can alternatively be embodied as a selectable pressure regulatingvalve, with which the opening pressure can, for example, be varied via acontrol device. A modulatable apparatus is thus available to supply theburner 18 with combustion air. Particularly in a start-up phase of theinternal combustion engine 12, the secondary air feed line 114 can beheld closed, a primary air mass flow of the atomization of the fuelbeing available via the primary air feed line 112. Thus, the atomizationof the fuel can be optimized with regard to the air volume available andthe design of the atomizing device 110.

In FIG. 3, an alternative embodiment of the apparatus for fuelpreparation 100 of a burner 18 is depicted. In this case, the secondaryair feed line does not open out directly in the combustion chamber 116but in the atomizing device 110. The atomizing device 110 comprises, forexample, a central nozzle tube, in which fuel and a primary air massflow are flowing. In addition, a casing tube which has openingssurrounds the central nozzle tube, the secondary mass flow being fedthrough said openings. The alignment of the openings can thereby be suchthat said secondary mass flow is subjected to a swirl. The flamegenerated in the process is thereby stabilized and the direction thereofis determined. An air flow for cleaning and cooling the nozzle can alsothen be maintained using said atomizing device 110 if the fuel feed andthe primary air feed is interrupted. The service life of such anatomizing device 110 is thereby increased.

A further exemplary embodiment of the apparatus for fuel preparation 100of a burner 18 is depicted in FIG. 4. The fuel supply 22 flows into theprimary air feed line 112, fuel being introduced into said primary airfeed line 112 via a suitable device. In this way, the fuel supply 22opens into an intake line of the air pump, for example, a rotary vanepump. The fuel is profoundly mixed with the air and the aerosol producedis supplied to the atomizing device 110 by means of a feed line 120.When needed, further combustion air can be delivered via the secondaryair feed line 114 across a pressure regulating valve 118, saidcombustion air mixing with the aerosol which was produced and beingignited as an ignitable fuel/air mixture in the combustion chamber. Itis furthermore shown in FIG. 4 that the fuel is supplied in meteredfashion, for example, by means of a controllable fuel pump or by meansof a fuel metering valve 122. A check valve is provided in the primaryair feed line 112. According to the invention, the several valves, whichinclude a pressure regulating valve, a metering valve and a check valve,can be combined into one component.

1. An apparatus for fuel preparation (100) for a burner (18) forregenerating a particle filter (16) of an exhaust gas aftertreatmentsystem comprising an exhaust gas passage (14) for discharging a hotexhaust gas flow from an internal combustion engine (12), wherein theapparatus for fuel preparation (100) comprises an atomizing device (110)for preparing the fuel, a combustion chamber (116), a combustion airsupply (24) and an ignition device (26), characterized in that thecombustion air supply (24) provides a primary air feed line (112) forpreparing the fuel and a secondary air feed line (114) for feedingcombustion air in order to produce an ignitable fuel/air mixture.
 2. Theapparatus for fuel preparation (100) according to claim 1, characterizedin that a pressure regulating valve (118) is disposed in at least one ofthe primary and secondary air feed lines (112, 114).
 3. The apparatusfor fuel preparation (100) according to claim 2, characterized in thatthe pressure regulating valve (118) is a mechanical pressure regulatingvalve.
 4. The apparatus for fuel preparation (100) according to claim 2,characterized in that the pressure regulating valve (118) is a controlvalve.
 5. The apparatus for fuel preparation (100) according to claim 2,characterized in that the combustion air supply (24) takes place via theprimary and the secondary air feed line (112, 114) when the pressureregulating valve (118) is open.
 6. The apparatus for fuel preparation(100) according to claim 5, characterized in that the primary air feedline (112) opens out into the atomizing device (110) and the secondaryair feed line (114) opens out into the combustion chamber (116).
 7. Theapparatus for fuel preparation (100) according to claim 5, characterizedin that the primary air feed line (112) and the secondary air feed line(114) open out into the atomizing device (110).
 8. The apparatus forfuel preparation according to claim 1, characterized in that fuel isinjected into at least one of the primary or secondary air feed lines(112, 114) and the fuel/air mixture produced is supplied to theatomizing device (110).
 9. The apparatus for fuel preparation (100)according to claim 1, characterized in that the combustion air supply(24) comprises a metering valve and the fuel is supplied in meteredfashion by a fuel metering valve (124), wherein a plurality of valves(118, 122, 124) is combined into one valve block.